EXCLUSIVE: Charlie Hunnam Thanks 'Incredible Girlfriend' After Ghosting Her for 5 Months for 'Lost City of Z'

Dating Charlie Hunnam isn’t as glamorous as it seems when he’s preparing for a role! The hunky Lost City of Z star, 36, went full method for his new role as isolated British explorer Percy Fawcett and opened up to ET about his girlfriend paying the price.

“It's a tough job trying to bring characters to life, so whatever you can do to limit the amount of actual acting and work you have to do on set,” he explained to ET’s Ashley Crossan at the Los Angeles premiere of the film. "A lot of the theme of this film is about the conflict of trying to do what we want for ourselves and our lives and bringing forth our personal aspirations and dreams for ourselves, and the conflict of how that affects the other people in our lives and our other responsibilities. So I just wanted to feel lonely and selfish so it worked. Thankfully, I have an incredible girlfriend who's very, very understanding and supportive of these silly whims that I have.”

And as the actor approaches 40 -- he turns 37 on Monday! -- he's looking forward to a building a more a stable life with her that probably doesn't involve five months of the silent treatment!

"I've got three more years til the big 4-0, I don't know, it feels significant, 40 feels really significant for some reason," he said. "It's just a number like any other but I gotta start having some kids, get married, start being sensible in my life."

While Hunnam himself shied away from calling his approach to the role“method," his co-star Sienna Miller praised him for it.

“He was very method in an amazingly impressive way,” Miller told ET. “He was starving himself half the time, he didn't speak to his [girlfriend] at all, and she's amazing. But he really buried himself into it and he has an intensity and I think it really resonates in the film but we had a lovely time because our dynamic in the film is lovely.”

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And Hunnam’s longtime love wasn’t the only one he distanced himself from throughout the filming process. His co-star, Robert Pattinson also got the cold shoulder. But now that filming has stopped, the pair can finally hang out as friends.

“It's nice actually to come out of that fog of the production and see that he's a lovely guy,” Hunnam said of Pattinson. “I can actually enjoy having a drink with him now and a hang.”

Hunnam noted that his decision to remain aloof on set was a calculated one on his part.

“There was something really exciting about the idea of not knowing someone, just letting a relationship exist exclusively on the screen and so I had decided that I wanted to do that,” he said. “But sometimes that can be alienating, and put another actor off of their work and I don't want to be that selfish. I mean, I'll be selfish in my personal life, but I got on and I started playing this game with Rob and he just started playing it right back. I think we both really understood what we were doing and there was no real contention or beef between us, that we were just playing the game.”

For more from Lost City of Z producer Brad Pitt, watch the clip below!